The present invention relates to an optical tracing system for measuring elevation of a reference body above a reference surface. The system of the invention is especially useful when attached to the blade or another part of an earth-moving, paving or grading machine which is being driven at normal working speed. The system detects a reference body such as a string line, cable, rod, curb or surface, measures and memorizes data regarding the initial elevation of the reference body above the ground, and thereafter continuously measures and compares measured values with the memorized data.
Elevation measurements are commonly performed in road construction operations and an ultrasonic range finder is normally used for this purpose. However, such a device requires some care in order to ensure accurate measurements, as it is adversely affected by wind, air temperature and temperature gradients. In addition, ultrasonic waves are reflected not only from the measured body, but also from the background, such as the ground beneath the body, which results in faulty measurements.
Optical tracers, based on the triangulation measuring technique, have been proposed in the past, e.g., in U.S. Pat. Nos. 4,456,826; 4,774,403; 5,129,725; 5,015,868 and 4,911,551. The first of these patents discloses a device based on rotating multi-facet mirrors, which is expensive and not durable if utilized in an open-air environment. In the other patents, a CCD array serving as a detector and a laser utilized as a light source are described. Such components are hardly compatible with road construction conditions, which, in general, require the low laser power dictated by eye safety regulations. These conditions include a large range of operation temperatures, below -20.degree. C. in winter and up to 55.degree. C. in summer in direct sunshine, and an extremely wide range of outer illumination conditions, from cloudy and rainy weather to bright sunshine, and also require the ability to measure elevation above different reference bodies (string line, cable, rod, curb, ground or passageway) wherein the type of reference body has to be automatically identified, as well as limited size and power consumption of the device used.